Our invention relates generally to apparatus for data transfer with magnetic disks such as those of the flexible type having a series of concentric tracks formed on one or both of these surfaces. More specifically, our invention pertains to an electric control system for use in or with such a magnetic disk apparatus, known also as a disk drive, for assuring the writing of data exactly on each predetermined track on the magnetic disk after the transducer head has been accessed thereto.
The usual practice in flexible magnetic disk drive art is to employ a stepping motor, which rotates in short and essentially uniform angular movements, in combination with a lead screw or like motion translating mechanism for transporting the transducer head radially of the magnetic disk from track to track thereon. Although the head may travel linearly from one track to another, it usually will not come to an immediate stop on the new track but will oscillate laterally thereof for some time before resting stably. We will refer to the time required for the linear travel of the head from one track to the next as the traveling time, and to the time during which the head oscillates or is unstable on the new track as the settling time. The sum of the traveling time and the settling time is what we call access time.
Should writing be commenced during the settling time, or before the end of the access time, then the written data would not subsequently be read correctly. The instruction manuals of some commercial disk drive models warn the user of this possibility. However, this warning is to no effect in some instances.
Usually, disk drives are not self contained; they are slave units under the control of host equipment typically comprising a central processor unit (CPU) having an associated controller. The host equipment may be programmed to commence writing immediately upon reception of index pulses, representative of the rotation and angular position of the magnetic disk, and of the identification signal from the disk drive. If the identification signal is delivered to the host equipment before the end of each access time of the head, writing will be commenced when the head is not yet stably positioned on the new track, resulting in the difficulties pointed out previously.
It may be contemplated to prevent the disk drive from delivering to the host equipment any data read from the disk until the head becomes stabilized on each new track. This solution will work, however, only in cases where the host equipment is programmed as above to start writing after reading that identification data. The controllers of some host equipment are constructed to set the disk drive out of operation if no data are read from the disk following the reception of the index pulses from the disk drive. The above suggested solution is not applicable to this type of apparatus.
Obviously, it would be easy for any magnetic disk specialists to overcome all the noted troubles if we were to design and manufacture each disk drive for a particular type of host equipment. The manufacturing costs of the disk drives of such diverse designs would be much higher than those of universal devices that lend themselves to use with various types of host equipment.